The present invention relates to a method for absolute measurement of the geometrical or optical structure of an optical component and to a device implementing it.
The method according to the invention makes it possible to provide absolute measurement of a polished surface or the distribution of the refractive index of an optical component.
Measurement of polished surfaces and/or refractive index distribution finds many practical applications in industry. It is particularly useful in the ophthalmic field for checking and measuring ophthalmic lenses. It can also be used for checking or measuring molds, for example those used in manufacturing ophthalmic lenses.
In the checking of optical components, the use of the so-called Ronchi test employing phase detection has already been proposed.
As is known, the Ronchi test consists of inserting a grating of alternately opaque and transparent parallel lines at the point of convergence of the light waves originating from an optical component to be checked, and then analysing the component of the fringes which are then observable downstream thereof.
If we limit ourselves to the laws of geometrical optics, without taking account of diffraction phenomena, these fringes represent the direction of the light rays that constitute the waves concerned and are characteristic of the aberrations thereof. Their slope reflects the difference between the corresponding wave surface and a spherical wave surface, the center of curvature of which is situated in the plane of the grating. It is consequently sufficient to measure this slope at all points on the optical component to be checked, this being achieved in practice using phase detection, and then to carry out integration in order to find defects in the surface of said component.
An optical device enabling such a method to be implemented is notably described in the article "Fringe Scanning Ronchi test for aspherical surfaces" published in "Applied optics", volume 23, number 20 of Oct. 15, 1984, as well as in the article "Phase measuring Ronchi test" in this same periodical, volume 27, number 3 of Feb. 1, 1988. Generally, this optical device comprises, arranged along an optical axis, light emitting means suitable for constituting a coherent point light source, a control station designed to carry the optical component being checked, a Ronchi grating, receiving means adapted to receive the observable image downstream of said Ronchi grating, and computing means designed to exploit this image, using phase detection.
Other optical component checking devices have been proposed. Thus, the use of two associated, substantially parallel Ronchi gratings has been proposed, the moire fringes obtained on a screen by coherent light transmission or reflection on the surface under study then being observed. Just like the Ronchi test, the moire patterns obtained give an indication, in terms of slope, of the differences between a plane theoretical wave surface and the wave surface obtained by transmission or reflection at the surface under study.
An optical device employing this principle is for example described in "Moire Deflectometry--Ray Tracing Interferometry" by I. Glatt and O. Kafri, published in "Optics and Lasers in Engineering" 8 (1988), pages 277 to 320. Such a device typically comprises a collimated light source which is transmitted or reflected by the surface to be analysed, to a pair of Ronchi gratings, the image being projected onto a mat screen. Qualitative analysis of the moire patterns obtained, when compared to the expected patterns, enables aberrations to be located. Varying the distance between the gratings allows quantitative measurements to be made together with calculation of the variations in measured wave surface compared to the plane theoretical wave surface.
Finally, in the device described in French patents 2 647 912 and 2 647 913, a map of the slopes of the actual surface to be measured is obtained by deflectometry using phase detection and, after subtracting the map of the nominal theoretical surface from the map of the the actual surface slope obtained, it is simple to obtain a map of the slopes of the defects, thus enabling the actual surface to be reconstructed, using integration. This known device, even if it enables a known ophthalmic lens to be checked with a high degree of accuracy or, more generally any known surface, does nevertheless suffer from the disadvantage of being limited to measurement of variations between a real surface and a theoretical surface. In other words, it implements a relative method involving prior knowledge of the theoretical shape of the surface to be measured.